Outdoor Shower/Indoor Bathroom Problem...

Having all sorts of problems in this rental house we just moved into! It was empty for two years before we moved in, apparently... anyway, the big thing is leaks under the floors.

The one that I'm most concerned about is in our 2nd bathroom. There's no actual, overt water visible, but I can see the porcelain tile is turning color (soaking up water) and the 'stain' keeps getting bigger! We don't use shower in there, and the 'stain', as it were, is actually up against the wall that abuts the outdoor shower (which we use daily) on the other side.

What needs to happen to find out where the water's coming from? Walls / floors ripped open? There is a drain on the lanai where the shower is... I'm wondering if it's just not draining properly? I don't know, I'm pretty lost when it comes to plumbing issues, lol! ;-> And the owner's a (non-handy-type) old guy who lives 1200 miles away....

Also, this same thing (tile sopping up water) is occurring in the laundry room, but we believe it's coming from a leaky washer. After fixing the washer, is there a possibility of mold/mildew forming under that tile?

Nothing you have posted is enough to give us any indication of what the real problem is or how to cure it. You need a plumber on site who can test the entire system to determine whether it is a water pipe, drain pipe, or shower wall/base leak.